The fabrication of fused and flexible optical fiber bundles for the purpose of transmitting electromagnetic signals from a signal-receiving end to a signal-emitting end of the optical fiber bundle is an evolving technology.
Optical fiber bundles arranged in one and two-dimensional arrays of signal transmitting optical fibers can be used as "optical fiber interconnects" to connect one and two-dimensional arrays of electromagnetic signal sources (e.g., photodiode arrays) and detectors in order to eliminate data transmission bottlenecks that arise in advanced digital systems communicating over short distances, for example. The practical implementation and usefulness of optical fibers as "optical fiber interconnects" requires optimum alignment of the optical fibers at at least one of the signal-receiving and signal-emitting ends of the fibers within an optical fiber bundle. Precision alignment of the optical fiber ends into an array at at least one common end of an optical fiber bundle is important in order to achieve efficiency in signal reception by, and transmission through, the optical fibers in the optical fiber bundle. An important measure of precision is how far the position of the axial center of any given fiber deviates from its ideal position. That is, how precisely are the desired distances between the axial center of a particular fiber maintained with respect to the axial centers of its neighboring fibers in an array (i.e., "center-to-center spacing"). The problem may also be expressed in terms of how closely the positions of the axial centers of the fibers approximate their objective, ideal lattice positions. Despite extensive, industry-wide research and development efforts, precision alignment of optical fiber ends, for these and other applications, has proved to be a vexing and elusive obstacle.
In some cases, fixtures have been used in an attempt to achieve precisely aligned arrays of optical fiber ends. Specifically, individual fibers have been adhered to array blocks or other fixtures where they are held in place with epoxy, for example. Some versions of this technique have called for individual fibers to be feed through individual holes in an array block and then fixed in place with epoxy or placed and adhered in channels created by micromachining or lithography, for example. The effectiveness and the degree of precision achievable by such techniques standing alone are limited by the precision with which the channels can be formed and spaced and by the precision with which the optical fibers can be placed and set therein.
Representative of recent attempts to achieve precise two dimensional arrays of optical fibers is a method for aligning optical fibers described by C. V. Cryan in a paper entitled "Two-dimensional multimode fibre array for optical interconnects," Electronics Letters, Vol. 34 No. 6, Mar. 19, 1998, p. 586. In the method described by Cryan, each optical fiber within the array is fabricated using the rod in tube method, as is known in the art. The fiber preform comprises a central core rod, a concentric inner cladding tube over the central core rod and a concentric outer cladding tube that fits over the inner cladding tube. Each fiber preform is then drawn into fused fiber rods using industry-standard equipment and methods. The diameter and cross-section of each fiber rod is carefully monitored and controlled during the drawing stage.
The resulting fused fiber rods are then aligned into a lattice configuration to form a modified array preform. According to Cryan, reasonably precise alignment of the fiber rods is possible due to their large diameter and flexural rigidity. The array preform is then drawn, fused and segmented to yield a multiplicity of array rods. During the draw stage of the array preform, the concentric outer glass tubes of the individual fiber rods within the array fuse with those of their neighboring rods thereby filling interstitial gaps within the array. The end faces of the array rods are polished perpendicular to the optical fiber rod axes to produce a straight, rigid optical fiber array. If desired, the rigid array can then be shaped in a heated former to produce a rigid data conduit.
Cryan further teaches that, when the outer concentric cladding tube of each optical fiber is an acid soluble glass, a flexible array can be fabricated by removing the acid soluble glass around each fiber along the length of the array rods, while a fused section is preserved at each end. As is known in the art, selective glass removal may be achieved by masking the ends of the array with an acid resistant coating and then leaching the array in a dilute acid solution to remove the unmasked glass between the ends.
Ultimately, Cryan's method involves the heating and drawing of bundled optical fibers to create a fused optical fiber bundle. As a matter of general observation, variability in the heating and drawing process renders very precise fiber alignment at the array face extremely difficult to achieve with regularity. Specifically, observation and experimentation indicate that the deviation from an ideal array is usually caused more by the relative positions of the optical fibers then by imperfections, inconsistencies or deviations in the fiber diameters and geometries themselves. This is because, during the heating and drawing process, the claddings of the constituent fibers in the bundle become very soft and molten-like allowing the fiber core to drift from its ideal lattice position.
For the foregoing reasons, there is a need for a reliable method and apparatus in which optical fibers are precisely aligned in an array.